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What inhibits the inhibitors? In the hippocampus, each memory trace is encoded by a specific subset of pyramidal cells. The other pyramidal cells must be actively excluded from the memory encoding process by inhibition, which is done by selective dendrite-targeting interneurons. Szőnyi et al. found that γ-aminobutyric acid–releasing (GABAergic) cells located in a small region in the brain stem called the nucleus incertus project to the hippocampus. The nucleus incertus again is innervated by several regions that respond to salient stimuli. Its GABAergic cells preferentially inhibit the dendrite-targeting interneurons in the hippocampus. The nucleus incertus is thus a central mediator between brain regions that are highly responsive to salient stimuli and the hippocampal circuitry involved in memory formation. Science, this issue p. eaaw0445 A brainstem regulatory mechanism for the selection of hippocampal neuronal assemblies during contextual learning is described. INTRODUCTION Associative learning is essential for survival, and the mammalian hippocampal neurocircuitry has been shown to play a central role in the formation of specific contextual memories. Contrary to the slow, neuromodulatory role commonly associated with brainstem systems, we discovered a highly specific, spatiotemporally precise, inhibitory ascending brainstem pathway that effectively controls hippocampal fear memory formation. Pyramidal neurons of the dorsal hippocampus CA1 region pair multisensory contextual information (see the figure, panel A, CA3) with direct sensory-related inputs (see the figure, panel A, EntCx). Each memory trace is encoded by a specific subset of pyramidal neurons. Remaining pyramidal cells must be actively excluded from the given memory-encoding process by direct dendritic inhibition, which is executed by somatostatin-positive (SOM) dendrite-targeting interneurons. SOM interneurons are activated by excitatory inputs from the medial septum (MS) upon salient environmental stimuli. Previous models suggested that the subset of memory-forming pyramidal cells escape this dendritic inhibition only by a stochastic, self-regulatory process, in which some SOM interneurons become inactive. However, we hypothesized that this process must be regulated more actively, and SOM interneurons should be inhibited precisely in time, on the basis of subcortical information; otherwise, underrecruitment of pyramidal neurons would lead to unstable memory formation. RATIONALE γ-aminobutyric acid (GABA)–releasing inhibitory neurons of the brainstem nucleus incertus (NI) seemed well suited to counterbalance the activation of SOM interneurons, as they specifically project to the stratum oriens of the hippocampus, where most SOM cells arborize. Using cell type–specific neuronal tract tracing, immunoelectron microscopy, and electrophysiological methods, we investigated the targets of NI in the mouse hippocampus, and in the MS, where excitation of SOM cells originates. We also used monosynaptic rabies tracing to identify the inputs of GABAergic NI neurons. Two-photon calcium imaging was used to analyze the response of GABAergic NI fibers to sensory stimuli in vivo. Finally, we used in vivo optogenetics combined with behavioral experiments or electrophysiological recordings to explore the role of the NI in contextual memory formation and hippocampal network activity. RESULTS We discovered that NI GABAergic neurons selectively inhibit hippocampal SOM interneurons in the stratum oriens both directly and also indirectly through inhibition of excitatory neurons in the MS (see the figure, panels A and B). We observed that NI GABAergic neurons receive direct inputs from several brain areas that process salient environmental stimuli, including the prefrontal cortex and lateral habenula, and that these salient sensory stimuli (e.g., air puffs, water rewards) rapidly activated hippocampal fibers of NI GABAergic neurons in vivo. Behavioral experiments revealed that optogenetic stimulation of NI GABAergic neurons or their fibers in hippocampus, precisely at the moment of aversive stimuli (see the figure, panel C), prevented the formation of fear memories, whereas this effect was absent if light stimulation was not aligned with the stimuli. However, optogenetic inhibition of NI GABAergic neurons during fear conditioning resulted in the formation of excessively enhanced contextual memories. Optogenetic stimulation of NI GABAergic neurons also changed memory encoding–related hippocampal theta rhythms. CONCLUSION A role of NI GABAergic neurons may be fine-tuning of the selection of memory-encoding pyramidal cells, on the basis of the relevance and/or modality of environmental inputs. They may also help filter nonrelevant everyday experiences (e.g., those to which animals have already accommodated), by regulating the sparsity of memory-encoding dorsal CA1 pyramidal neurons. NI GABAergic neuron dysfunction may also contribute to dementia-like disorders or pathological memory formation in certain types of anxiety or stress disorders. Our data represent an unexpectedly specific role of an ascending inhibitory pathway from a brainstem nucleus in memory encoding. Nucleus incertus (NI) activation prevents memory formation. NI GABAergic neurons regulate contextual memory formation by inhibiting somatostatin interneurons (SOM IN) directly in hippocampus (HIPP) (A) and indirectly through inhibition of their excitatory inputs in the medial septum (MS). Pairing optical stimulation (B) with aversive stimuli (C) eliminates fear memory formation, whereas control mice display normal fear (freezing) after exposure to the same environment a day later (D). Hippocampal pyramidal cells encode memory engrams, which guide adaptive behavior. Selection of engram-forming cells is regulated by somatostatin-positive dendrite-targeting interneurons, which inhibit pyramidal cells that are not required for memory formation. Here, we found that γ-aminobutyric acid (GABA)–releasing neurons of the mouse nucleus incertus (NI) selectively inhibit somatostatin-positive interneurons in the hippocampus, both monosynaptically and indirectly through the inhibition of their subcortical excitatory inputs. We demonstrated that NI GABAergic neurons receive monosynaptic inputs from brain areas processing important environmental information, and their hippocampal projections are strongly activated by salient environmental inputs in vivo. Optogenetic manipulations of NI GABAergic neurons can shift hippocampal network state and bidirectionally modify the strength of contextual fear memory formation. Our results indicate that brainstem NI GABAergic cells are essential for controlling contextual memories.